Kienholz: How much does a player improve after his age-20 season?

From Alex Kienholz at Beyond the Box Score on October 15, 2013:

The past few seasons there's been no shortage of impressive seasons by 20 year olds. In particular I'm talking about Mike Trout, Bryce Harper, and Manny Machado. That got me thinking about other players that played significant time as a 20 year old, and ultimately how their careers ended. In July I did something similar, but in that case I looked at 20 year pitchers, That article can be read here.

In this study I took all players with at least 200 plate appearances in their age 20 season, since 1975. The metric of choice was Wins Above Replacement, provided by FanGraphs. After collecting all of the data I ended up with a total of 35 players that met the aforementioned criteria. These players ranged everywhere from Ken Griffey Jr. to Clint Hurdle.